Condensed Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Statement Of Cash Flows [Abstract]
Income taxes, refunds	$ 18	$ 51,904

The amount of cash received during the period as refunds for the overpayment of taxes.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3536-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586

Name:	us-gaap_ProceedsFromIncomeTaxRefunds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration